Conventional X-ray tubes commonly use rotation of the anode to dissipate heat. Such rotation is distinctly different from rotation of the cathode about its central point which will affect the X-ray intensity distribution of the X-ray tube focal spot, as found at the radiographic image receptor.
A typical X-ray tube is characterized by a focal spot which has a non-uniform, double peaked intensity distribution. This type of distribution may result in either a double image or a sharp false image, and is often the limiting factor to the diagnostic usefulness of certain radiographic magnification procedures involving objects of subfocal dimensions. Focal spots with a Gaussian-like intensity distribution have been advocated not only because of the need for increased high frequency information, but also, because of the need for elimination of spurious resolution.
Electron beam focusing techniques have been used in conventional X-ray tubes to produce small focal spots with Gaussian-like intensity distributions. Radiographs have been shown which illustrate the diagnostic effect of such focal spots in small vessel angiography. It has been pointed out, however, that in the case of microfocal spots obtained with focusing voltage techniques, the focal spot size depends not only upon the focusing voltage, but also upon the tube voltage. The required type of intensity distribution can be maintained only with a restricted set of tube voltages.
It has been theorized that the non-uniform double peaked intensity distribution results from the difference in pathways that electrons coming from the back and sides of the cathode filament follow in reaching the anode. These differences in pathways create a non-uniform distribution of electrons impinging on the anode and thus, ultimately create the non-uniform focal spot distribution of X-rays.
The required type of intensity distribution can be maintained, in these known conventional arrangements, only with a restricted set of tube and focusing voltages. Thus, the operational flexibility of X-ray apparatus using such tubes has been limited. These shortcomings are distinct disadvantages.